The invention relates to a device for a contactless transmission of data between a data transceiver and at least one portable data carrier.
Contactless identification systems with inductive transmission of energy and data from a data transceiver (CODxe2x80x94Card Operating Device) onto a portable data carrier via an alternating magnetic field are used, for example in the case of chip cards. Such a system is described by Klaus Finkenzeller in the book entitled xe2x80x9cRFID-Handbuchxe2x80x9d [RFID (radio frequency identification) manual], Carl Hanser Publishers Munich 1998, pages 183 to 205. Operating the chip cards with the data transceiver requires a given power to generate the alternating magnetic field.
The antenna for generating the alternating magnetic field is generally an arbitrarily shaped conductor loop. The conductor loop has an inductive reactance under the usual operating conditions. In the normal case, this inductive reactance is compensated through the use of a matching circuit composed of resistors, capacitors and coils, and the antenna is thereby tuned to a resonant frequency. Tuning to the resonant frequency means that the inductive reactance has a value equal to zero, and that essentially only the loss resistances remain as impedance.
It is necessary to match the antenna to resonance when the antenna and the power source are connected in the data transceiver via a cable of unknown length. In order to remain independent of the cable length, the antenna and the power source must be matched to the characteristic impedance of the line. The matching of the antenna to the characteristic impedance is described, for example, in U.S. Pat. No. 5,241,160.
Operation in resonance is usually also employed when the antenna is connected directly to the power source. This is the case, for example, whenever the antenna and the power source are mounted on the same printed circuit board.
When no portable data carrier is located in the alternating magnetic field, a maximum current flows in the case of tuning the antenna to resonance. This maximum current entails a maximum magnetic field.
Consequently, tuning to resonance also causes high losses in no-load operation. No-load operation is understood in the following as the operating state of the data transceiver in which no portable data carrier is located in the active field of the data transceiver. The active range of the data transceiver is that distance from the data carrier to the antenna at which the alternating magnetic field is still just sufficiently large to enable data to be exchanged with the portable data carrier.
The alternating magnetic field, which is generated by the current flowing through the antenna, is mostly much stronger than actually required, particularly in the case of data transceivers of short range (so-called closed coupling systems).
If a portable data carrier is located in the active field, the latter reacts on the antenna. This reaction or feedback can be seen in the occurrence of an additional impedance in the antenna circuit of the data transceiver. If the antenna is tuned to resonance, this reaction is a maximum, that is to say the additional impedance occurring in the antenna circuit effects a reduction in the antenna current, and thus simultaneously a reduction in the magnetic field. This reaction or feedback increases with an increasing coupling between the antenna and the portable data carrier. The coupling is generally larger for smaller distances between the antenna and the portable data carrier. In the most unfavorable case, as the antenna is approached, the portable data carrier can reduce the current through the latter because of the reaction to such an extent that an adequate power supply between the data transceiver and the data carrier is no longer possible.
In order to ensure operation of the data carrier in the active field of the data transceiver, it is therefore necessary, when the antenna is tuned to resonance, to maintain a correspondingly high current in the no-load operation. This ensures that, when a data carrier is in the active range, the magnetic field strength is still adequate, despite the reaction, to preserve an adequate power supply for the data carrier. This means that the current through the antenna is sufficiently strong when a data carrier is brought into the active field very high losses occur as a consequence of the high power for generating the alternating magnetic field in no-load operation.
The mode of operation described in the case of configuring the antenna for resonance is even more disadvantageous when the system is configured for a plurality of data carriers. The reaction or feedback of a plurality of data carriers to the antenna circuit is then multiplied in accordance with the number of the data carriers located in the active field. The power source providing the power for the data transceiver must therefore be even larger, which results in an increase of the required space and in high costs.
It is accordingly an object of the invention to provide a device for a contactless transmission of data which overcomes the above-mentioned disadvantages of the heretofore-known devices of this general type and which consumes only little power for generating an alternating magnetic field for a given active range.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for a contactless transmission of data between a data transceiver and at least one portable data carrier, including:
a data transceiver and a portable data carrier;
the data transceiver includes a transmitting device for producing a first signal of a given frequency; a receiving device for receiving a second signal of a given frequency; an impedance-transformed antenna connected to the transmitting device and to the receiving device; and a power supply connected to the transmitting device;
the portable data carrier includes a data carrier antenna for one of receiving and transmitting an induced signal; a circuit configuration connected to the data carrier antenna for processing the induced signal and for producing a signal to be transmitted to the impedance-transformed antenna of the data transceiver; the impedance-transformed antenna of the data transceiver having a total impedance composed of a reactance and an ohmic loss resistance, the reactance being not equal to zero when the portable data carrier is not inductively connected to the impedance-transformed antenna of the data transceiver.
In other words, according to the invention the antenna of the data transceiver is configured in terms of the total impedance, that is to say in a circuit which performs an impedance transformation, such that the reactance is non-vanishing when no portable data carrier is inductively connected to the antenna. In other words, this means that the antenna is not tuned to resonance if no portable medium is located in the active range of the data transceiver.
In this way, the introduction of a reactance into the antenna reduces the current which has to be maintained in the case of a no-load operation by comparison with the prior art. This entails lower low-load losses. For this reason, the power supply can be less powerful and/or of smaller dimensions. There is no need to provide an expensive cooling device. A further advantage is that, due to the reactance already present in no-load operation, upon the introduction of a data carrier, the reaction or feedback in relation to the data carrier on the antenna can be reduced, in other words it is used in an advantageous way. This has the consequence, for example, that the current reduction due to the introduction of the data carrier into the active field is substantially smaller than the current drop in the case of a data transceiver according to the prior art. It is thereby possible to avoid the problem that the current in the antenna can be cut off due to the introduction of a data carrier into the active field.
The reactance can be both, an inductive-type reactance or a capacitive-type reactance. It is even possible to set the reaction of the cards to the antenna so as to produce a defined increase in the antenna current for a given layout case, for example three data carriers which must still operate in a given range of x cm. The reactance must be appropriately dimensioned for this purpose. The reactance can be determined only through the use of a complicated simulation calculation. The power required by the data transceiver for generating the alternating magnetic field or for the power supply of the data carriers can be substantially reduced by the proposed measure of specifically introducing a reactance into the antenna circuit.
In a preferred embodiment of the invention, it is also possible to combine the two solutions. This means that the antenna of the data transceiver can have both a reactance, that is to say is therefore not tuned to resonance, and an additional loss resistance. As a result of this additional loss resistance, which is located, for example, between the antenna and the matching circuit, the current is reduced in no-load operation of the data transceiver, and thus the low-load losses are reduced. The reaction of the data carriers located in the magnetic field of the data transceiver on the antenna is, in turn, reduced by the provision of the additional loss resistance. The reaction caused by the occurrence of an impedance in the antenna circuit of the data transceiver is certainly also noticeable in a reduction in the current through the antenna which is associated with a reduction in the magnetic field strength. However, the current drop is substantially less pronounced than in the case of a data transceiver according to the prior art. Due to the low current drop upon the introduction of a data carrier into the active field, the required strength of the alternating magnetic field remains high enough, although the power supply of the data transceiver can be small, and the transceiver thereby also provides a lower current through the antenna.
In accordance with another feature of the invention, an additional resistance is provided between the impedance-transformed antenna and the transmitting device and/or the receiving device.
In a further embodiment, the data transceiver, whose antenna has an inductive reactance and an additional resistance provided between the antenna and the transmitting device or the receiving device, has a matching circuit device between the antenna and the transmitting device or the receiving device.
In a further embodiment, the various solutions, with which the power reduction in the power supply of the data transceiver is effected through the use of a reactance in the antenna, have a connection of known length which interconnects the antenna and the transmitting device and/or the receiving device. There is no need to provide a matching circuit when use is made of a connection, for example a cable, of known length.
With the objects of the invention in view there is also provided, a device for a contactless transmission of data between a data transceiver and at least one portable data carrier, including:
a data transceiver and a portable data carrier;
the data transceiver includes a transmitting device for producing a first signal of a given frequency; a receiving device for receiving a second signal of a given frequency; a line of unknown length, the line having a characteristic impedance; a data transceiver antenna connected to the transmitting device and to the receiving device via the line of unknown length; a matching circuit configuration connected between the data transceiver antenna and at least one of the transmitting device and the receiving device, the matching circuit configuration having at least one resistance limiting a factor Q for matching a total impedance to the characteristic impedance of the line, and the matching circuit having an additional loss resistance;
the portable data carrier includes a data carrier antenna for transmitting and for receiving an induced signal; and a circuit configuration, connected to the data carrier antenna, for processing the induced signal and for producing a signal to be transmitted to the data transceiver antenna.
In other words, the matching circuit has an additional loss resistance in addition to the resistor which serves to limit the factor Q. The matching circuit configuration, which includes at least a resistor for limiting the factor Q as well as capacitors and/or coils, is introduced between the antenna and the transmitting device and/or the receiving device. The matching circuit configuration serves to match the total impedance to the characteristic impedance of a line of unknown length. The same effect and the same advantages as described above are achieved with this measure. Again, the reaction of the data carriers, located in the field of the data transceiver, on the antenna is thereby reduced. The consequence is that it is possible to maintain in no-load operation a current which is reduced by comparison with the prior art.
A preferred solution is achieved by introducing a reactance into the antenna of the data transceiver. The lowest currents in no-load operation can be achieved through the use of this solution. Of course, it has been assumed in this case that the boundary conditions are the same for all variants. This means that the data transceiver is configured for a specific number of data carriers, and the active range in which data can be exchanged between the at least one data carrier and the data transceiver assumes a prescribed distance from the antenna.
The advantage of all the above-described solution principles is that the output stage of the power supply (AC voltage source) can be dimensioned for lower powers due to the reduction in the required antenna current. This results in a reduction in costs. A further advantage is that, by comparison with the prior art, the circuit configuration requires no additional circuit elements between the power supply and the antenna. The components are only differently dimensioned.
The power transmission via an inductive coupling utilizes only the magnetic near field of the antenna. However, this also unavoidably entails emission of electromagnetic waves. The emitted power is directly proportional to the square of the antenna current in this case. This means that the reduction in the antenna current simultaneously reduces the effectively emitted power. This facilitates the observance of norms or standards which require a limited emission. These problems will be explained later.
In the solution in which an additional resistance is inserted between the antenna and the matching circuit, the sum of the resistance relating to the limitation of the factor Q and the additional loss resistance is calculated using the following formula:   R  ≈                              k          min                ⁢                  (                                    k              max                        ⁢                                          (                                  2                  ⁢                  π                  ⁢                                      xe2x80x83                                    ⁢                                      f                    0                                                  )                            2                        ⁢                          L              F                        ⁢                          L              T                                                  2        ·                  "LeftBracketingBar"                      B            T                    "RightBracketingBar"                      -                  R        F            .      
A calculation using this formula, which takes account of the configuration of the data transceiver (number of data carriers, active range) yields the maximum power reduction in no-load operation. The data transceiver then operates reliably in all operating states. A drop (=cutoff) of the current to unfavorable values in the antenna of the data transceiver cannot occur given dimensioning using this formula.
The resonant frequencies of the portable data carriers can be dimensioned either to be equal to the prescribed operating frequency of the data transceiver, or else to be greater than or less than the prescribed operating frequency. In a preferred embodiment, the resonant frequency of the portable data carriers is configured to be higher than the prescribed frequency. The circuit configuration located on the portable data carrier is supplied with voltage by the induced signal which is produced upon the introduction of the data carrier into the active field of the data transceiver in the data carrier antenna. Use is made for this purpose of the resonant increase by a series resonant circuit composed of a capacitor and the inductor of the data carrier antenna. The mode of operation will be explained more precisely later in conjunction with the figures.
The circuit configuration on the portable data carrier can be configured as an integrated semiconductor chip or as a discrete circuit. Chip cards, security labels for various objects, or else identification devices can be understood as portable data carriers. However, it is also conceivable for the portable data carriers to be installed, for example, in motor cars, so that tolls may be paid for the use of specific roads.
The data transceiver according to the invention and the data carrier antenna are capable of coupling with one another in a range of between 0 and 1 m. The data transceiver is preferably configured such that the antennas are coupled in a range of between 0 and 1 cm. In the case of data carriers in the so-called CD-1 format (check cards, defined in ISO), the data carrier and data transceiver are coupled in a range of between 0 and a few centimeters (e.g. 4 cm). Such data transceivers are called closed coupling systems.
In a configuration of the data transceiver for a range from 0 to 15 cm, the systems are called proximity systems. In the range of between 0 and 1 m, the data transceivers are termed vicinity systems. The field of application of the invention is, however, not limited theretoxe2x80x94rather, it depends on the relative sizes of the antenna and data carrier.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for contactless transmission of data, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.